Evaluation of Setup Uncertainties for Electron Boost Field Treated Using Compression Device for Breast Cancer Irradiation

Purpose: To investigate uncertainties of patient setup for electron boost field using an in-house breast compression device for breast cancer irradiation.Method: Ten early stage breast cancer patients with deep seated tumor beds treated with post-lumpectomy radiation were randomly selected. The patients underwent resimulation and treatment with the compression device to decrease the depth of tumor bed. A cone beam computed tomography (CBCT) scan was performed for each patient before treatment and fused with the simulation CT. Four surgical clips located in superior, inferior, left and right of tumor bed were identified in both CT and CBCT data sets for each patient. Set-up uncertainties were estimated based on differences in the positions of the clips between scans relative to the marked isocenter. Results: For each patient, all clips shifted consistently in the same direction superior-inferior (SI) and left-right (LR) direction, but not in the anterior-posterior (AP) direction because of the perturbation of the compression device. 50% of clips (20 out of 40 clips) shifted anteriorly, 60% shifted inferiorly, and 70% shifted left. Clip shifts in AP direction were smaller than in SI and LR direction. The 55% of clips shifted >1 cm, 18% of clips shifted > 1.5 cm, and only one clip shifted > 2cm which was out of the intended isodose coverage. The average shift of 4 clips for each patient in LR direction was correlated with the breast volume (R2=0.406), but the average shifts of 4 clips in AP and SI direction were not correlated breast volume (R2>0.02).Conclusion: For most patients whose breast boosts were treated with a compression device, a 2cm margin was sufficient to ensure target coverage based on set-up uncertainties observed in this study. A larger margin might be necessary for electron boost fields for patients with large (volume >2000cc) and/or pendulous breasts.